In a mobile communication system, a signal transmitted from a base station propagates through various paths (also called “propagation paths”) by reflection, diffraction, or scattering and reaches a mobile station. Here we explain it with an example illustrated in FIG. 19. FIG. 19 is a diagram for explaining multipath. In the example illustrated in FIG. 19, a signal transmitted from a base station 91 propagates through paths PT1 to PT3 and reaches a mobile station 92. In this manner, a plurality of paths through which one signal propagates is called multipath.
When a signal propagates through multipath like the above example, an arrival time at which the signal reaches a mobile station differs according to path. Here we explain it with an example illustrated in FIG. 20. FIG. 20 is a diagram illustrating an example of a time-domain power profile. In the example illustrated in FIG. 20, a signal which has propagated through the path PT1 reaches the mobile station first, then a signal which has propagated through the path PT2 reaches the mobile station, and then, a signal which has propagated through the path PT3 reaches the mobile station. Therefore, like an example illustrated in FIG. 21, a signal that the mobile station receives significantly fluctuates in amplitude in a frequency domain.
Therefore, some mobile communication systems select a frequency band having good channel quality in communication and establish communication using the selected frequency band. Specifically, a base station allocates a frequency band having good channel quality out of frequency bands that can be used in communication with mobile stations (hereinafter, referred to as a “system bandwidth”) to each mobile station. Then, the base station transmits allocation information, which is information of the allocated frequency band, to each mobile station. And then, the base station and the mobile station communicate with each other using the allocated frequency band. Such a process for allocation of a frequency band is sometimes called frequency scheduling.
Here we explain it with an example illustrated in FIG. 22. Incidentally, in the example illustrated in FIG. 22, the base station 91 shall communicate with three mobile stations 92a to 92c. Like the example illustrated in FIG. 22, a frequency band A has good channel quality in communication between the base station 91 and the mobile station 92a. Therefore, in the example illustrated in FIG. 22, the base station 91 allocates the frequency band A to the mobile station 92a, and communicates with the mobile station 92a using the frequency band A. For the same reason, in the example illustrated in FIG. 22, the base station 91 communicates with the mobile station 92b using frequency bands B and D, and communicates with the mobile station 92c using frequency bands C and E.
When such frequency scheduling is performed, a base station acquires quality information indicating the channel quality from a mobile station or measures the channel quality by its own self. Here we explain it more specifically with reference to FIGS. 23 and 24. FIG. 23 is a diagram illustrating an example of a common pilot in a downlink. FIG. 24 is a diagram illustrating an example of a pilot for channel-quality measurement in an uplink.
For example, when the base station performs downlink communication with mobile stations, as illustrated in FIG. 23, the base station uses the entire system bandwidth to transmit a pilot signal to the mobile stations. Each of the mobile stations measures the channel quality using the pilot signal received from the base station, and transmits quality information indicating the measured channel quality to the base station. Then, the base station performs frequency scheduling on the basis of the quality information received from the mobile stations.
Furthermore, for example, when uplink communication is performed, like the example illustrated in FIG. 24, each of the mobile stations transmits a pilot signal for channel-quality measurement to the base station. The base station measures the channel quality on the basis of the pilot signal for channel-quality measurement, and performs frequency scheduling on the basis of the measured channel quality.
Incidentally, in the example illustrated in FIG. 24, the mobile station 92a transmits a pilot signal for channel-quality measurement using the entire system bandwidth at pilot-signal transmission intervals. On the other hand, the mobile stations 92b and 92c transmit a pilot signal for channel-quality measurement by changing a frequency band at pilot-signal transmission intervals. Namely, the mobile stations 92b and 92c cover the entire system bandwidth by transmitting the pilot signal several times by frequency hopping.
As an example of a transmission scheme for performing the frequency scheduling described above, OFDM (Orthogonal Frequency Division Multiplexing) is well known. In the case of using the OFDM, the base station designates a frequency band to be allocated to a mobile station in bitmap format. Then, the base station transmits allocation information, which is information on the frequency band allocated in bitmap format, to the mobile station.
Specifically, like an example illustrated in FIG. 25, the system bandwidth is divided into resource blocks by the minimal bandwidth allocated to a mobile station (hereinafter, referred to as the “minimal allocated bandwidth”), and with respect to each of the divided resource blocks, whether or not to allocate a resource block to the mobile station is designated by a bit code. Incidentally, in the example illustrated in FIG. 25, a resource block with “1” set in a bitmap denotes that the resource block is used in communication; a resource block with “0” set in the bitmap denotes that the resource block is not used in communication.
Namely, in the example illustrated in FIG. 25, with respect to the mobile station 92a, the base station allocates resource blocks B11 to B13 out of resource blocks B11 to B21 to the mobile station 92a. Furthermore, the base station allocates resource blocks B14, B15, and B17 to the mobile station 92b, and allocates resource blocks B16 and B18 to B21 to the mobile station 92c.     Non-patent document 1: 3GPP, TS36.211 V8.5.0 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)    Non-patent document 2: 3GPP, TS36.213 V8.5.0 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 8)
However, the above-described conventional technology has a problem that it is difficult to finely allocate a frequency band to each mobile station. Specifically, to narrow a frequency band to be allocated to each mobile station, the minimal allocated bandwidth is reduced; however, in this case, the size of allocation information is increased. For example, in the case of using the OFDM as a transmission scheme, the number of bits designated in bitmap format is increased. Therefore, if a frequency band is finely allocated to each mobile station, an amount of control information transmitted and received between the base station and the mobile station is increased.